I wouldn't call a parent at work unless I needed immediate assistance. I never call a client to update. I will often email a parent mid day if it is not urgent, but something that I don't want to bring up in front of little ears.

If a child had a tantrum like that with me I would let the client know that I will count this as a one time event, however I'm just not equipt to deal with that type of behaviour on an ongoing basis. If it happens again then we will have to consider alternate care for the child.

I'm not sure I know what issues you're having with the 6 year old, but that's what I'd do about the 3 year old.

And I think that's very honest of you to give back the deposit if you choose to term immediatly. I don't like to term mid week unless it's really bad.